1. Field of the Invention
The present invention relates to a process for reducing the width of a flat metal product by rolling wherein the product is passed through at least one roll train having width rolls and thickness rolls. In another aspect the invention relates to a process of hot rolling steel strip from a slab.
2. Description of the Prior Art
Very large quantities of steel strip are produced at the present time, for use in the manufacture of consumer goods, especially motor vehicles. This strip is conventionally hot rolled from slabs to the desired thickness in a finishing rolling train containing a large number of roll stands at which thickness is reduced but width is substantially not altered. Since different customers have different demands for the width of the strip which they buy, it is a major problem in operating such a roll train (which is a highly expensive plant and has a very large throughput capacity) to arrange the feeding of hot slabs of the desired widths in the desired order to the beginning of the roll train. Such slabs come from several sources: (a) slabs from continuous casting machines (the continuously produced strand must be cut before being supplied to the roll train because the output rate of a single continuous casting machine is very much lower than the capacity of the roll train), (b) slabs from a slabbing mill in which cast ingots are rolled into slabs and (c) cold slabs brought in form storage or from an outside source of supply.
All such slabs are passed through a reheating furnace prior to entering the finishing rolling train, in order to bring them to the desired temperature and to ensure that they are at uniform temperature all through their volume. Hitherto, slabs have generally been cold when supplied to the reheating furnace, which makes the arrangement of the slabs in the desired order fairly easy. However, with a view to saving energy, it is nowadays highly desirable that slabs from a continuous casting machine or from a slabbing mill should pass directly into the reheating furnace with the minimum of delay. It is not convenient to control in detail the width of the slabs produced in a slabbing mill in dependence on the width requirements for the finishing rolling train.
Also, it is highly impractical to shut down a continuous casting machine frequently in order to change the width of the slabs which it produces. An appreciation of this problem in a continuous casting machine is shown in U.S. Pat. No. 3,358,358 which is dated 1967 i.e. before high energy costs became a serious problem. This patent proposes that the continuous strand produced is reduced to the various desired widths by edge rolling of the strand while longitudinal tension is applied to the strand between spaced horizontal roll stands. The purpose of application of longitudinal tension is to reduce the tendency towards a "dog-bone" cross sectional shape which has hitherto arisen during edge rolling (in the "dog-bone" shape the outer portions of the rectangular section are considerably thicker than the middle portion).
Specifically, U.S. Pat. No. 3,358,358 proposes the use of a special width reducing roll train which has two horizontal roll stands (i.e. thickness roll stands) which apply the longitudinal tension, and a plurality of width roll stands located between the two thickness roll stands. This process is designed for use with a strand which has been produced by continuous casting and the width rolling occurs immediately after temperature equalization has been carried out in a furnace. The object of the process is to use the substantial width reduction that can be obtained when width rolling is carried out under high traction forces, in order to enable a continuous casting machine to produce slabs of desired width (less than the actual casting width) without the need for changing the casting mould or for cutting the continuous casting into discrete slabs and then reducing their width by rolling them by conventional methods before finishing.
However, the process suffers from a number of disadvantages in practice. Firstly, as it is performed continuously after the casting process, each continuous casting machine used will require an individual width reducing roll train. Secondly it is nowadays desirable in practice that the substantial width reduction in the high traction width reducing roll train is followed by a finishing roll train. As mentioned, the capacity of a finishing roll train is far greater than the production capacity of a continuous casting machine and thus for optimum operation a plurality of continuous casting machines are required to supply the finishing roll train. The product must therefore be cut before entering the finishing roll train unless a plurality of finishing roll trains are used which would be very expensive and uneconomic. However the width reducing roll train of U.S. Pat. No. 3,358,358 cannot be used economically with discontinuous slabs, because the full traction force is not exerted on the head and tail of the slab as it passes through the width reducing roll train. This results in a large quality variation in the slab at its ends. The length of the width-reducing roll train is so great (as mentioned above) that the resultant waste in each slab would be such as to render its use with slab highly uneconomic. Therefore the process is apparently suitable only to be operated continuously so that an individual width reducing roll train is required for each continuous casting machine, which is expensive. Furthermore the intermediate produced rolled can only be obtained by continuous casting. The disadvantages of this process are therefore such that it is not thought to have been used other than experimentally.